1. Field of the Invention:
The present invention relates to a cooling roll suitable for use in the process of producing thin metal tape directly from a molten metal by the twin-roll method or single-roll method.
2. Description of the Prior Art:
There is a known method of producing thin metal tape directly from a molten metal. According to this method, a molten metal is allowed to jet from a nozzle and the jet is brought into contact with the surface of a roll running at a high speed for cooling and solidification. This method is classified into the single-roll method and twin-roll method according to the number of rolls used.
The roll for producing quenched thin metal tape is made of high speed steel or sintered hard alloy as disclosed in, for example, Japanese Patent Laid-open No. 119650/1981. The conventional roll, however, has a disadvantage that it cannot be used for long-time operation, because when the roll surface gets hotter than 600.degree. C. in the production of thin metal tape thinner than several millimeters, the thin metal tape may stick around the roll or seize to the roll surface, or cracking may occur on the surface of the roll.
For instance, in a crystalline strip such as a high silicon steel strip of 500 mm width, heat capacity .DELTA.H to be cooled, including sensible and latent heat, amounts to 350,000 cal/sec, which is estimetated as so times as large as compared to that of an amorphous metal strip.
The heat value .DELTA.H can be calculated by the following equation.
.DELTA.H={.rho..C.)(T.sub.m -T.sub.s)+.alpha...rho.}.W.d.v
where:
.DELTA.H: heat value
.rho.: density
C : specific heat
T.sub.m : melting point
T.sub.s : solidified temperature
.alpha.: latent heat
W : width
d : thickness
v : roll velocity
And it is noted that .DELTA.H for an amorphous metal lacks .alpha...rho. in the above equation and on the other hand .DELTA.H for a crystalline metal includes .alpha...rho. which is 10 times or more larger than .rho..C.(T.sub.m -T.sub.s).
To eliminate this disadvantage, there was proposed in Japanese Patent Laid-open No. 77918/1982 a quenching roll made of copper alloy such as Cu-Zr or Cu-Be having a high thermal conductivity and high strength. This roll is in general use at present.
However, the roll of copper alloy still suffers from the disadvantage of being subject to hair-cracking or microcracking in the continuous production of thin metal tape thinner than several millimeters by the twin-roll method. This trouble may occur when the operation is continued to process a molten metal in excess of 500 kg. The hair-cracked roll causes the molten metal to penetrate into the cracking resulting to stick around the roll, which leads to the unavoidable discontinuance of operation owing to breakout and so on.
In order to solve this problem, the present inventors proposed in Japanese Patent Laid-open No. 116956/1983 a cooling roll for producing quenched thin metal tape of high silicon steel, said roll having a coating layer of nickel plating or nickel alloy plating. This cooling roll is superior in wear resistance and is immune to the seizure of thin metal tape. Nevertheless, it is still subject to hair-cracking when it cools a large amount of molten metal continuously.
The surface coating of the cooling roll is not necessarily effective, depending on the material of surface coating and the conditions of operation, in preventing the seizure or sticking of thin metal tape in the production of thin metal tape thinner than 1 mm, with the cooling roll running at a high peripheral speed. This is particularly true of iron rolls and some copper alloy rolls having a low thermal conductivity, because the sensible and latent heat to be removed may be estimated too large for such rolls.
On the other hand, copper alloy, rolls having a high thermal conductivity decrease in hardness at high temperatures and hence wear and/or roughen after operation for a long time.
In the meantime, rolls for the twin-roll method are liable to deformation at high temperatures (500.degree. C. or above) because the two rolls are pressed against each other to perform rolling. Deformation takes place at the part where the two rolls come into contact with each other. The deformed rolls fluctuate the thickness of the thin metal tape and roughens the surface of the thin metal tape.
To prevent the seizure or sticking of thin metal tape, the roughening and wear of roll surface, the deformation of roll, the fluctuation of metal tape thickness, and the roughening of metal tape surface, there was developed a cooling roll of copper alloy having a high thermal conductivity and a high strength at high temperatures. Even this cooling roll is subject to intercrystalline cracking (or hair cracking) resulting from the thermal fatigue at high temperatures under high pressures, if the copper alloy is of precipitation hardening type (such as Cu-Be, Cu-Zr-Cr). Therefore, this cooling roll does not withstand continuous operation for a long time.